Working on live
telephone circuits can be a problem. You just can't connect a normal
scope probe to the circuit while it is tied to the phone line.

Neather side of a phone line is ground. When a phone or other device
is 'offhook', one side floats at about -22 volts and the other side about
-30 volts.

This probe allows connecting to such 'floating' circuits to a normal scope
input. As seen in the circuit, it is based on using an optocoupler.
The one shown in the diagram is a dual unit, but I had a pile of them
here, so thats what I used. Because this device responds to both
polarities, an external diode was added to prevent negative voltages from
producting an output.
The input has 2 ranges: 1 to 1, and 10 to 1. There is no need for
a power switch because with no input, virtually no power is drawn from
the 12 volt (keyfob) battery. Also, no power is drawn from the 1.5
volt (watch) battery.

Because of variations in individual optocouplers, you should select a
value for R3 to get the best results.

Now for the bad news.....

This device is enherently non-linear. The small battery on the input
side helps compensate by giving an initial bias to the optocoupler's input
LED. Also, presenting a 10k load to some circuits will cause addional
error in the output. The non-linearity is not too bad from 0 to
5 volts, above that, it is not very usable.

Nonetheless, this little gadget has already 'paid for itself' in making
some timing and voltage measurments of how a circuit responds to a telephone
ring signal.